Image sensors are typically formed on a semiconductor substrate. For example, Complementary Metal Oxide Semiconductor (CMOS) image sensors are typically formed on a silicon substrate. CMOS processes to fabricate photo-sensitive pixels comprising photodiodes are well known in the art and described in various patents by OmniVision Technologies, such as U.S. Pat. No. 7,355,228 and U.S. Pat. Pub. 20070072325, the contents of which are hereby incorporated by reference.
In a Front-side Illuminated (FSI) arrangement the front-side of the image sensor receives the input light after the light passes through other front-side portions, such as a micro-lens, color-filter array, infrared filter, and metal interconnect regions. The micro-lens focuses light into each pixel. The color-filter array filters a specific color of light into each pixel. CMOS-based image sensors are typically sensitive to infrared radiation. As a result, certain wavelengths of infrared light have the potential to degrade sensor detection of visible light. Consequently, an infrared filter is typically used in CMOS image sensors designed to detect visible light.
However, a disadvantage of a FSI image sensor is that it is difficult to achieve a high fill factor, where the fill factor is the percentage of the pixel area sensitive to light. In particular, the metal and polysilicon lines of the interconnection regions of the CMOS circuitry can block some of the light from reaching individual photodiodes, reducing the fill factor.
Referring to FIG. 1, in a Back Side Illuminated (BSI) image sensor the image sensor receives light through a backside of the substrate supporting the image sensor. The fill factor of a BSI image sensor can be high and in some cases approach 100%. However, a BSI image sensor typically requires that the substrate be thinned to reduce optical absorption. For example, in some cases the substrate is thinned down to a silicon thickness of less than ten microns after front-side processing of the image sensor is completed.
A BSI image sensor typically has the backside coated with a transparent conductive coating (TCC) formed from Tin doped Indium Oxides (ITO). The transparent ITO coating is used to establish a uniform voltage bias to the substrate portion of each individual photodiode of the pixel array. A voltage bias provides several benefits, such as improved quantum efficiency and signal-to-noise ratio for some sensor configurations. ITO is a conductive oxide typically formed by vacuum sputtering and annealing. ITO has a high transparency to the visible spectrum and a low resistivity. An infrared filter is formed over the ITO layer, followed by a color filter array and a micro-lens.
However, an ITO TCC also has several undesirable characteristics. Referring to FIG. 2, a typical BSI image sensor fabrication process includes a front-side image sensor fabrication step 205, including epitaxy, diffusions, and metallization to form a pixel array. The backside of the silicon wafer is thinned in step 210. A shallow layer of N or P type dopants are implanted and activated in the backside in step 215. The ITO film is vacuum sputtered on the backside in step 220. The ITO film requires a moderate temperature anneal, as illustrated in step 225. An infrared filter is formed in step 230. The color filter array (CFA) and micro-lens (ML) are formed in step 235. Note that ITO TCCs require the thinned image sensor wafer to go through a thermal cycle at moderate temperatures. The thermal cycle required for the annealing step 225 is high enough (e.g., 400 degrees Celsius in some cases) that degradation of the image sensor is a potential concern. This degradation concern arises both because of the mechanical fragility of the thinned substrate and because the annealing temperatures are high enough to potentially affect front-side features. Moreover, ITO films used in image sensors are comparatively rigid and inflexible, which may stress the thinned substrate. Additionally, the cost of forming transparent conductive coatings using ITO has increased dramatically in recent years.
Therefore, in light of the above-described problems with ITO in BSI image sensors, the apparatus and method of the present invention was developed.